Blender 2.8 Scripting for a simple button
Pre or Append to the header.

With all panels and headers in blender you can append or prepend a draw method to them.



Test script prepends a splash screen button to topbar

import bpy

def draw(self, context):
    self.layout.operator("wm.splash")

bpy.types.TOPBAR_HT_upper_bar.prepend(draw)


To find the class to "leach" onto, with developer extras on, right click and view source. With the naming convention, TOPBAR pretty self explanatory, HT it's a header type. Hence using autocomplete in python console is a way to find contenders